A type of shaft coupling structure has been known that couples a shaft and a sleeve, into which the shaft inserted, to each other by fitting a tolerance ring between the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve. In such a shaft coupling structure, when the torque transmitted between the shaft and the sleeve exceeds an allowable value, which is determined by the maximum frictional force between the tolerance ring and the shaft or the sleeve, the tolerance ring rotates while slipping on the shaft or sleeve, so that the transmitted torque is limited to be less than or equal to the allowable value.
Patent Document 1 discloses one example of a tolerance ring used in such a shaft coupling structure. The cylindrical main body of this tolerance ring has protuberances, which protrude radially outward. Lubricant for limiting wear caused by slip rotation of the tolerance ring can be stored between the protuberances and the shaft, which is arranged radially inward of the main body.